Groundwater is often contaminated by volatile organic compounds, particularly chlorinated and aromatic hydrocarbons. Conventional separation technologies, such as distillation and liquid--liquid extraction, are not applicable because of the large volumes of water that must be treated.
Pump-and-treat methods of remediating nonaqueous phase liquids in groundwater are generally not satisfactory, primarily because the "pump" part of the technology cannot efficiently remove the contaminates from the aquifer. The organic compounds from the nonaqueous phase liquids slowly leak into the groundwater over a long period of time. Several attempts are being made by remediation companies to use surfactants to more efficacious removal of the nonaqueous phase liquid components by emulsifying them with an aqueous surfactant solution. Similarly, organic contaminants, which have been found to adhere strongly to soil particles, can be removed from soil more easily when a surfactant solution is pumped through the contaminant region. This process is called surfactant flushing. Although the "pump" part of technology is improved this way, the "treat" part becomes more complicated. Generally, two established methods can be used to dispose of the resulting emulsion:
(1) air or steam stripping, which is not a useful solution because the VOCs, which are then in air or condensed in a liquid form, still must be disposed of; and
(2) bioremediation, which is unproven for VOCs, and, even were it successful, would require large installations.
Pervaporation is a technique wherein volatile organic compounds from an aqueous medium are preferentially transported across a thin membrane film. The source side of the membrane is wetted with the aqueous liquid, while vacuum or a sweep gas is used on the sink side of the membrane. The VOCs are collected from the sink side by condensation. It is customary to expect a concentration factor of 1000 or more. Most VOCs are hydrophobic, so that a hydrophobic membrane must be used. In contrast to other membrane filtration processes, pervaporation works according to a solution-diffusion mechanism. The membrane itself must be porous for pervaporation to work. In microfiltration or ultrafiltration, for example, porosity is the key to preferential transport, and the flux rate depends upon molecular size. In pervaporation, molecular interaction rather than molecular size is the determining factor.
For pervaporation to be economical and efficient, ultra-thin non-porous hydrophobic films of appropriate polymers, such as polydimethylsiloxane or polyether imide block copolymer, must be deposited onto a highly porous support matrix. The preferred configuration of the membrane modules is a hollow fiber.